I'm a little surprised that Vise grips didn't work for you. I've had no trouble using them to break rust-welded studs...which is what is going to happen (99% certainty). That's the cleanest removal method, when the entire knob unit has to be replaced. When the associated handlebar is scrap metal, too, it's the preferred method.
I'd clamp the f****d handlebar securely in a vise, get the Vise grips as tight as possible on the stud end, then
reef on that mofo (technical term
)... i.e. apply a ton of torque. The M8 stud should break-off, and pretty cleanly, at the handlebar threads...where the metal has been weakened by rust. If you're unable to make this method work, then the next best solution would be carefully cutting the head of the stud. After that, the handlebar can be pulled straight out.
FYI, even if you have the proper spanner to unscrew the steel nut from the triple tree, it won't do you any good. Only way to remove the nuts without major scarring is with a spanner socket, or wrench. Without it say goodbye to the handlebar nut(s).